text:
What are typical values of the lift-to-drag ratio for normal small aircraft?
Hi I am trying to do a rough estimation of forces acting on an aircraft wing and would like to know what kind of lift to drag ratio is normal. The exact profile hasn't been chosen yet, but the aircraft weighs less than 1800 kg and has a wingspan of around 13 m. The cruise speed is roughly 70 m/s.
As I understand, the maximum load factor to consider is 3.8, so the maximum lift is about 67,000 N; but how would I go about getting an estimate for the drag?

The maximum L/D values for existing aircraft span a wide range, from 7.5 for the Zodiac Ch701 with floats to 21.5 for the Berkut 360. Source: Light Sport and General Aviation Airplane Comparison and Harloff Performance Factor. This comparison comprises 93 types, and that is normally where a drag calculation starts: You find a similar type and use that as a baseline.
For the wing drag you calculate the lift distribution for several angles of attack (try XFLR5 for this) and get the induced drag for the weights and speeds that belong to those angles. Now you need to know your airfoils and add their drag. Next, add increments for the interference with the fuselage, for fairings, gaps and probes using data from this book. Yes, it is more than half a century old but physics hasn't changed since then.
Drag is normally not the biggest in-plane load on a wing but lift at high angle of attack. If you need to calculate loads for a structural design, make sure you don't omit that particular load case. Don't be surprised: That lift has a strong forward-facing component.
If you are new to this, try to calculate several of the aircraft for which data exists. Only by doing this you can be sure that what you do gives good results. Drag calculation needs some experience and is, when done well, a lot of work.

"That lift has a strong forward-facing component." I thought it was rearward facing?

@DKNguyen: Not in the body-fixed coordinate system. At high angle of attack lift is still about orthogonal to the flow direction which now has a noticeable angle with the body-fixed system. This creates the most important load case for wing in-plane loads.

Oh wait, you're talking about the lift component. I was thinking about the aerodynamic force.

@DKNguyen Relative to the aerodynamic system, the aerodynamic force is always rearward-facing. That is the contribution of both friction and induced drag. But for wing in-plane loads this needs to be converted into the body-fixed system.


text:
Radiotelephony certificates in Germany: to get AZF do i need BZF first?
I've been asked by LBA (German Civil Aviation Authority) to acquire the general radio telephony certificate (AZF) in order to convert my FAA ATP to a German EASA ATPL.
Can I get the AZF immediately or do I have to get the restricted radio telephony certificate (BZF) first?

Can you explain the abbreviations?

The agency responsible for issuing  aeronautical radio licences is the Bundesnetzagentur (the Federal Network Agency).  At their website you can find the requirements for these licenses.
They have five types of licenses:

BZF II > Beschränkt Gültiges Sprechfunkzeugnis II für den Flugfunkdienst
Radiotelephone certificate II for the aeronautical radio service with limited validity
BZF I > Beschränkt Gültiges Sprechfunkzeugnis I für den Flugfunkdienst
Radiotelephone certificate I for the aeronautical radio service with limited validity
BZF E > Beschränkt Gültiges Sprechfunkzeugnis E für den
Flugfunkdienst
Radiotelephone certificate E for the aeronautical radio service with limited validity
AZF > Allgemeines Sprechfunkzeugnis für den
Flugfunkdienst
General radiotelephone certificate for the
aeronautical radio service
AZF E > Allgemeines Sprechfunkzeugnis E für den
Flugfunkdienst
General radiotelephone certificate E for the
aeronautical radio service

These licenses have the following areas of applicablity.

BZF II - radiotelephony within the Federal Republic of Germany in German according to visual flight rules
BZF I - radiotelephony in German and English according to visual flight rules
BZF E - radiotelephony in English language according to visual flight rules
AZF - radiotelephony in German and English according to visual and instrument flight rules
AZF E - radiotelephony in English according to visual and instrument flight rules

Furthermore, website states the requirements for obtaining the licenses. In case of the AZF and AZF E, they state:

Voraussetzungen für den Erwerb eines AZF oder AZF E
Bewerberinnen und
Bewerber müssen mindestens 18 Jahre alt sein oder spätestens drei
Monate nach der Prüfung das Mindestalter erreichen.
Das AZF und das AZF E sind nur mit einer Zusatzprüfung zu erwerben, d.
h., dass für das AZF das Innehaben des BZF II oder BZF I und für das
AZF E das BZF E erforderlich sind.

Which translates approximately like this:

Requirements for the acquisition of an AZF or AZF E
Applicants must be
at least 18 years old or reach the minimum age at the latest three
months after the examination.
The AZF and AZF E can only be acquired with an additional examination,
i.e. holding the BZF II or BZF I is required for the AZF and the BZF E
is required for the AZF E.

So you must first acquire the BZF (I or II) to acquire the AZF. If you want to acquire the AZF E, you must have the BZF E.

source: Stack Exchange

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text:
What is the willingness of non-AME physicians to perform BasicMed exams?
May 1st, 2017, BasicMed rolls out and for certain operations and certain pilots BasicMed promises a much more permissive and yet preventive oriented approach to medical approvals for pilots.
A concern is that one's physician may not issue BasicMed.
From AAFP's newsletter, where BasicMed was overviewed for Academy members (mostly Family Practice Physicians), I have extracted the following excerpts.

In an interview with AAFP News, Cowl stressed that physicians should
  understand FAA medical guidelines as they pertain to potential
  liability.
"There is the potential for liability to physicians, in part because
  the pilots most likely to use the alternative qualification pathway
  may be the very people who need the standard flight physical," said
  Cowl.
...
"At that altitude and type of airframe, these planes are sharing the
  airspace with paying commercial passengers. As physicians, we need to
  maximize the health of our patients and promote aviation safety," he
  said.
Simply put, "This comprehensive medical checklist is not a summer camp
  physical," said Cowl. Physicians should familiarize themselves with
  FAA guidelines, communicate with aviation medical examiners in their
  communities and understand the liability they take on when they put
  their medical license number on the FAA form.
Cowl acknowledged that some family physicians may already perform
  commercial truck and bus driver examinations that are similar in
  nature to pilot exams. For those physicians, a certain comfort level
  may already exist.
However, he added, "Physicians who do these exams need to be cognizant
  of the 'ask' -- and that is to perform a comprehensive exam and then
  sign an attestation statement that says, 'Based on my comprehensive
  exam, this person is safe to fly.'"

So I asked my non-pilot family practice doc whether he would be doing BasicMed exams.  "Absolutely not.  Truck drivers are one thing, but flying planes and the liability potential is too great for me to stomach."
I asked another FP doc, and she told me that she would avoid doing the exams, because as she understands them, they require a comprehensive physical that has higher complexity than other physicals she performs, and along with it there is an unknown liability potential, which is new to the risk market and she doesn't like being a guinea pig.
Then I talked with a former student of mine, who is an emergency medicine attending, and holds a Private Airplane-Instrument certificate.  He has been tracking BasicMed, because it is relevant to him.  He hauls his family around at vacation time in his Saratoga, and BasicMed meets his requirements. "Honestly, I will just see (my AME) and get a regular third class medical.  I don't know anyone who wants to touch BasicMed if they are in private practice.  If you get someone working for a hospital clinic, it might be different."
So my question is: Is there any preliminary data on the willingness of physicians to perform BasicMed exams, and in particular those who are not AMEs or pilots?
Addendum #1
https://generalaviationnews.com/2017/05/18/basicmed-is-creating-conversation/
An update article, after the start of BasicMed.

It's a good question, but considering that the checklist has only been available for a few days it's probably far too early to expect any actual data. If you skim a few forums you'll probably get the impression that there seem to be more doctors saying no than yes, but it could simply be that disappointed pilots just post more than satisfied ones.

I just got back from my annual physical and my doctor did the BasicMed exam as part of it and signed me off without a problem. It was a total non-issue. I'm on a Special Issuance for an eye condition so this is going to save me a lot of hassle.

I tried going to my GP for a BasicMed exam, and he refused for liability reasons.  I called several AMEs, who told me they won’t do it either, but the last referred me to a non-AME who happens to be a pilot. He signed me off that day with no issues.
I would suggest using local pilot groups on social media to find out what doctors in your area are known to do it, rather than being a guinea pig. That would have saved me a few weeks of frustration.

I have Kaiser insurance and tried really hard to get them to conduct my physical, to no avail. I was thwarted by some degree by the fact that my regular physician is not seeing patients right now, but my suspicion is that she may not have done it either. However, I have also heard of Kaiser members who did convince their physicians, so apparently there is some wiggle room. I ended up going to a clinic that does the DOT exam after asking if they would also do basic med - the doc turned out to be a pilot himself. Saved just a few buck compared to an AME.
As to why some folks would do basic med rather than a third class, at a certain age the third class is only good for a couple of years, while the basic med exam is good for five. That's why I elected to go with Basic Med. (I read recently that BM won't allow flight outside of the US, including Mexico or Canada, so that's something to keep in mind if you fly in those areas...)

My normal PCP performed mine, no problem, just like I’ve have for racing exams and sports physicals.   Only my experience but nothing in the wording gave her any pause

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text:
Wing fuselage joint in small aeroplanes
I know that there are bolts to connect the centre wing boxes of the fuselage and wings in large aircraft, but what are the methods of connecting the wings and fuselage of small aeroplanes? I’m mostly curious about spars that go from wingtip to wingtip.

(Shortened) video of the wings being removed on a Cessna 152. It shows the wing bolt locations.

@Suryetto, if you think this question is not a duplicate of the linked one, the correct procedure is to answer here in the comments or to edit your question to make the difference more prominent, not flag the automated comments.

Why would a small GA aircraft be significantly different in construction method than a large commercial passenger liner? Just because it's small doesn't mean they forgo bolts and use superglue instead...

" I’m mostly curious about spars that go from wingtip to wingtip."  The spars in my Cessna Cardinal do not go from wingtip to wingtip. The wing spars connect to a carrythru spar that runs across the top of the cabin.  Looks like a big aluminum I-beam.  Here's a  shot of one from Cardinalflyers.com https://community.cardinalflyers.com/uploads/db0560/optimized/1X/d5bba206bf5eb9150e24b57978bf648c92724c20_2_999x750.jpeg  You can't see the big studs that are used to attach the wing to the ends of tho.

The post is NOT a duplicate of the answer which talks about airlines. My airplane is a low wing homebuilt, a PL-2 http://pazmany.com/wp/?cat=15 which has a one-peice wing from tip to tip.The fuselage simply sits on top of the wing and is attached with a bolt at each corner that joins the front and rear spars to the adjacent fuselage frames. This is a structurally superior arrangement insofar as the heavy spar beam root joints are eliminated and is lighter, but you are stuck with a very unwieldy 28 ft long all-of-a-piece wing if you want to take the wing off. So the other method is more common.


text:
Application of Bernoulli Energy Conservation
In what cases does the static pressure decrease when the velocity increases in a flow.
And in what cases does the static pressure remain constant whilst the velocity and stagnation pressure increases?
Please answer with respect to Bernoulli's equation: Po1 = Ps1 + 0.5*rho*v1^2
For example in a closed circuit wind tunnel to what extent does the fan convert static pressure to dynamic pressure and to what extent does it just add energy by increasing the dynamic pressure.

I'm voting to close this question as off-topic because it does not seem to be about aviation. This is a pure fluid dynamics questions, which belongs on physics.stackexchange.

Also looks like a homework question.

I disagree with closure. This isn't some esoteric non-Newtonian fluid dynamics question. It is readily "applicable" to wind tunnel as mentioned in the OP.

@Jimmy The requirements for closing a question are not being about "esoteric non-Newtonian fluid dynamics". It is just not aviation related. I think, the fact that you can apply it to wind tunnels does not change that.

@CrossRoads, arguably anything posted here could be "homework" without full context of why the question is being asked. For your consolation I am not asking this because I am required to hand in this information to some one else. It has just been nagging me for a while. My attempt to make it more understandable for you may make it seem as though a teacher set it. In this case I am the teacher, teaching myself.

@Bianfable, I am writing this question as I am analysing the construction of a wind tunnel. Wind tunnels perform experimental research on aviation models,  and fluid dynamics makes planes fly, that was my thought train. If I posted in the wrong place, I will correct that now. Thank you for the feed back.

@Von-Karmen you should edit the question to highlight what you put in your comments

Bernoulli's equation holds only if there is no energy source in the system. Since the fan adds energy, Bernoulli's does not apply across the fan; in the context of 1-D flow, pressure changes discontinuously across the fan.
Due to conservation of mass (no mass source for a simple fan), airspeed cannot change discontinuously across the fan interface. Therefore, right after the fan interface, both the static pressure and the total pressure have increased.
For more information, refer to this tutorial.

I would like to disagree with regards to the reason why Bernoulli's doesn't apply across the fan.

Firstly, I would like to disagree with regards to the reason why Bernoulli's doesn't apply across the fan.
Bernoulli's relies on three assumptions, the flow must be:

steady
inviscid
incompressible

The fan breaks Bernoulli's because it is not inviscid or steady. It is my error to ask in terms of Bernoulli. Instead it should be the steady flow energy equation.
I think I should be asking instead, what are the mechanisms by which the flow can alternate static and stagnation pressure? Perhaps that is a more esoteric physics question.

@Von-Karmen No sir. Conservation of energy must hold for streamline to exist. Navier Stokes in the conservation form inherently assumes conservation of momentum and energy. If you add a source to your control volume, Bernoulli breaks down.

Yes, you are right. But that doesn't help answer the question.

"If you add a source..." this is why I revised my answer to the steady flow energy equation.

@Von-Karmen I can only answer what you have posted. If you have something else in mind, please modify your OP.

@Von-Karmen See my modified answer. This is my best understanding of the intent behind your question.


text:
Why did this delta wing generate a lower Cl at a higher wind tunnel velocity?
I'm an aerospace engineering student,and just did an aerodynamics lab in which we measured the lift & drag of a delta airfoil inside a wind tunnel between 0 and 40 degrees, in 5 degree increments, at two flow velocities (subsonic, M< 0.5). For some reason, the lift co-efficient is .25 lower throughout the entire curve in the higher velocity run compared to the lower velocity run, despite the drag co-efficient s being identical until the stall region (which begins at 30 degrees and completely separates at 35). Why is this? Considering the nature of the lab, this seems what I should be discovering. I have a theory that it has something to do with stronger vortices generated by faster flows, but wouldn't that INCREASE lift by dropping the pressure on top of the wing further?
In addition, another oddity: the L vs D curve increases SHARPLY between 0 and 10 degrees for both flows, but then the slope decreases after that and stays steady until the stall region. Why is that, and is that specific to delta wings?

Did you non-dimensionalize with the correct speeds?

Do you have pictures of the models?

Did you actually calculate these points and plot them, or is this a computer generated result?  Raw data should show much higher lift and drag for higher airflow speed. It seems, in equalizing the drag plot, you've discovered you get better lift to drag at a lower airspeed.

The increase in the lift curve slope at low angle of attack is quite normal. At 10° you see the effect of vortex lift kicking in. Yes, that is specific to delta wings.

Attached and vortex lift over angle of attack (picture source). This is taken from the excellent Cambridge Core article on vortex lift and these particular diagrams are from E.C. Polhamus: A concept of the vortex lift of sharp-edged delta wings based on a leading-edge suction analogy, NASA TN D-3767, December 1966. They show lift over angle of attack, but lift over drag would look quite similar, also with a kink once vortex lift sets in.
The offset between the two speeds is harder to explain. This looks like an error in your angle measurement – there must have been different offsets between the two test runs. That there is an offset is obvious from the nonzero lift at zero angle of attack. Unless your delta wing had lots of camber, there should not be much lift at zero angle of attack.


text:
How high can I fly a model aircraft under 14 CFR 101 rules?
For a UAS operated recreationally, may altitude exceed 400 AGL?  AMA rules state in several places that flight above 400 AGL should be avoided. And close to airports there is a more firmly worded restriction. But away from an airport, and presumably in Class G, what are the altitude restrictions?
This question presumes 14 CFR 101 applies.
Addendum to question: FAA appears to be attempting to impose a 400AGL restriction on 101 operations, without regulation. See the amended AC: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91-57A_Ch_1.pdf, stating: Model aircraft operators should follow best practices including limiting operations to 400 feet above ground level (AGL).

This question is now obsolete due to changing regulations.  If you are still interested in the answer, you could re-ask, modifying the phrasing to say "under all currently applicable regulations" or "under all regulations in effect as of March 1 2020", etc.  Or if you are ONLY Interested in the effect of 14 CFR 101 on the answer, not other regulations, you could re-ask in a way that makes that clear.

There's no specific altitude restriction for model aircraft anywhere in part 101 (that I can see). Even within 5 miles of an airport you only have to inform the tower, per 101.41(e):

When flown within 5 miles of an airport, the operator of the aircraft
  provides the airport operator and the airport air traffic control
  tower (when an air traffic facility is located at the airport) with
  prior notice of the operation.

However, practically speaking there is an indirect altitude restriction in 101.1:

For purposes of this part, a model aircraft is an unmanned aircraft
  that is:
[...]
(ii) Flown within visual line of sight of the person operating the
  aircraft

In other words, you have to be able to see the model aircraft and that limits the altitude and distance you can fly it at. It's also worth noting that 101.43 is very broadly written:

§101.43   Endangering the safety of the National Airspace System.
No person may operate model aircraft so as to endanger the safety of
  the national airspace system.

Arguably, that also implies you shouldn't fly any higher than you really need to.

I tend to agree with you.  However, I am not really sure what you mean by "any higher than you really need to" because there can be reasons for flying higher.  For example, a bigger view. I get nervous with the concept of 700 ft drones, though.  As it is I am nervous with instrument approaches, especially non-precision approaches where the time spent at a lower altitude might be longer.

@mongo I agree, I just meant that the higher you go the more important it is to have a good reason (IMO).

One issue is the interpretation of when one endangers NAS. I haven't found many NTSB cases on that.

@mongo, I would be willing to bet that 101.43 is intentionally vague so that if there are any issues, they've got a catch-all regulation to get you with.

Well, I have a meeting with the FSDO to discuss this and some related things first thing in the morning.

FSDO was certain it was in the regs but is now scratching their heads, but said that they would violate on endangering the NAS for anyone flying above 400 feet.

@Pondlife, the following:  Model aircraft operators should follow best practices including limiting  operations to 400 feet above ground level (AGL). comes from https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91-57A_Ch_1.pdf. ACs are not regulatory, however, FSDO inspectors like to use the broad brush of ACs in enforcement actions. It seems that the FAA may be attempting to regulate without changing the regulations.

While FAR 101 is intended as advisory or "best practice", to get the complete picture you must go beyond FAR 101. FAR 107 places a strict altitude restriction and/or proximity of 400ft to taller structures. Unless operating under other provision such as "commercial operator" this is a mandatory restriction.

FAA FAR 107 states, "The maximum allowable altitude is 400 feet
  above the ground, and higher if your drone remains within 400 feet of
  a structure."

There are no specific altitude restrictions.
The reason for the 400 foot guidance is that powered aircraft are supposed to stay at least 500 feet above the ground or any structure on the ground. So, if you stay under 400 feet then you minimize chance of collision with a full-size aircraft.
The main exceptions would be crop dusters and flight paths into landing zones, such as airports, heliports, helipads and sea plane bases where aircraft will descend below 500 feet above the ground objects.

Helicopters have no such 500 foot rule. Worse is that helicopters are the ones usually flying low. See 91.119 https://www.ecfr.gov/cgi-bin/text-idx?rgn=div8&node=14:2.0.1.3.10.2.4.10

@Tyler Durden,  Do you have an authority how the 400 foot was initially derived?

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text:
How is US Class B airspace enforced in Canada?
Detroit, MI is the center of Class B airspace, however a good portion of that airspace within the inverted wedding cake is in Canada.  What are the mechanisms that allow US to define Class B airspace force and effect in Canada?
The VFR Terminal chart for reference:
https://aeronav.faa.gov/content/aeronav/tac_files/PDFs/Detroit_TAC_91_P.pdf

It's not US Class B airspace if it's in Canada.

The portion over Canada is not designated US Class B but Canadian Class C.
Note that Canadian Class C airspace definition is similar to US Class B.  In particular,  VFR aircraft must obtain clearance prior to entry.  A mode C transponder is required.
To add: airspace over Canada is generally managed by NAV CANADA.  Definition for the Canadian side of the Detroit Terminal Control Area (TCA) can be found in NAV CANADA's Designated Airspace Handbook, portions excerpted below (from page 78):

We can see that these definitions matches the eastern portions of the Detroit TCA exactly, e.g, the outer area altitudes from 6,000' to 10,000' bounded 30nm from the Detroit VOR (DXO).
The Detroit TCA airspace was modified a few years ago, which required coordination between the FAA and Nav Canada.  The following discussion from the US Federal Register (page 3307) may be of further interest:

Also, as noted in the NPRM, the
  eastern portion of the DTW Class B
  airspace area extends into Canadian
  airspace. The equivalent Canadian
  airspace to Class B airspace, as
  designated in the United States, is Class
  C airspace. NAV CANADA, the
  Canadian air service navigation
  provider, generally designates Class C
  airspace with a 12,500 feet MSL ceiling,
  however, has advised the FAA of its
  willingness to establish corresponding
  Canadian Class C airspace adjoining the
  FAA’s DTW Class B airspace with a
  ceiling of 10,000 feet MSL.
  Additionally, NAV CANADA advised it
  would make the Canadian Class C
  airspace action effective to match the
  effective date of this DTW Class B
  airspace modification action.

Who is the controlling agency for the space within Canada?

Welcome to aviation.SE! Can you include a source or reference for this information?

I've added a couple of references.  @mongo in Canada, air traffic / navigation services was privatized in the mid 90s (no longer controlled by a government agency) -- since managed by NAV CANADA, a not-for-profit corporation.

All airspace in each FIR (or UIR) is by default controlled by the corresponding ACC (US: ARTCC).  In the case of metro Detroit, it's Cleveland FIR/ARTCC on the US side and Toronto FIR/ACC on the Canadian side.
All other controlling agencies get their airspace by delegation from the ACC via an LOA.  Cleveland ARTCC delegates the airspace on the US side to Detroit TRACON, and Toronto FIR/ACC delegates the airspace on the Canadian side also to Detroit TRACON.  So, despite being a US agency, Detroit TRACON ends up controlling airspace in Canada as well.  This is common along international borders, even in places where the land border isn't as well agreed as the US/Canada one.

Would you kindly provide a source for this information?


text:
How is a 14 CFR 101 sUAS NOTAM filed for an unregistered aircraft?
FAA promotes the utilization of NOTAMs to announce sUAS operations, and requires it in 333 exemptions and some 107 waivers. However, sUAS (drones / model aircraft) under 14 CFR 101 have been determined to not require registration.
Drone NOTAM entries are not possible without a registration number.
How, without registering a 101 model aircraft, may one file a Drone NOTAM?
EDIT 15 June 2020:  Regulations have changed, and now Exception for Recreational Flyers requires that aircraft 250g or greater be registerd.  This addresses this question, as the question was relevant to former part 101 recreational flyers, which has been changed to now require registration.
However, aircraft below 250g do not require registration, and a pseudo identifier, created by the owner may be used as a registration identifier.
I will create an answer for this question reflecting the current regulatory environment.

The Flight Service website asks for an "Aircraft ID or Reg. No." and the tooltip says it's "a name you assign to the UOA. Examples: 2330012013, MYFLYER, UOA01" There's no indication that it needs to be an actual registration number, unless I missed something. Did you see that requirements stated somewhere else?

Liedos NOTAM processing has told people that a valid registration needs to be assigned. NOTAMs entered without a valid registration are reported by 101 users as being deleted, and not entered into the system. They also checked with their customer, the FAA NOTAM Office, which also told them, and then the 101 users that a registration number or N number was necessary. If you have a different authority, there are people who would love to hear about it, @Pondlife.

All part 107 aircraft must be registered, regardless of weight.  The question addresses the case of Part 101 flyers, who were exempt from registration.
Regulations have changed, and the Exception for Recreational Flyers now requires aircraft above 250g, used for recreational flights, to be registered.  Hence a registration number would be available for drone NOTAM filing.
In the case of aircraft below 250g, which are not required to be registered under the Exception for Recreational Flyers, an identifier may be offered by the filer of the NOTAM.  (eg: MONGO2025551212, which might be a name and phone number)

We file NOTAMs for sUAS at my military base by calling 1-800-WXBRIEF. Talk to the flight service specialist and they can complete the NOTAM for you.

Welcome to aviation.SE!


text:
In the US, what is the process for filing a DROTAM?
For drone pilots in the US, what is the process for filing a DROTAM?

This might be a better fit on [drones.se]

I’m voting to close this question because it belongs on Drones.SE.

I posted here because increasingly drones are used in commercial applications, and the issuance of DROTAMS while perhaps not of substantial interest to airliners and military jet jocks, are relevant to other sectors of aviation.  Local medvac guys brief on them daily.  Pipeline patrol and utility operator dispatchers are aware of them.  None of the three local ag operators look at them, but now that they are aware of them, they plan on briefing them daily.  It seems to be relevant to NAS and in general aviation safety, hence the posting.

This question seems appropriate both for aviation and drones/models. But it's not expected you ask "how to do..." without explaining your own attempt to file a Drotam and what difficulty you encountered, if any. To me the dedicated site which link was provided is self-sufficient a priori.


text:
What's a metering fix?
The Instrument Procedure Handbook says on page 3-18

"Arrival delays typically are absorbed at a metering fix. This fix is
  established on a route prior to the terminal airspace, 10000 feet or
  more above the airport elevation. The metering fix facilitates profile
  descents, rather than controllers using delaying vectors or a holding
  patterns at low altitudes."

I cannot get my head around this concept. So how does a metering fix work to absorb arrival delays? What's exactly a profile descent and how is it related to the metering fix? What merits does it have as compared to delay vectors and holds?

Definitions from this page.

Metering:

A method of time-regulating arrival traffic flow into a terminal area so as not to exceed a predetermined terminal acceptance rate.

Metering airport:

Airport adapted for metering and for which optimum flight paths are defined.

Metering fix:

A fix along an established route from over which aircraft will be metered prior to entering terminal airspace. Normally, this fix should be established at a distance from the airport which will facilitate a profile descent 10,000 feet above airport elevation [AAE] or above.

But behind these definitions, that's a whole domain under research and experimentation to shape the future of air traffic management.

Metering methods are currently designed to respond to traffic increases, resulting airports congestion, and requirement for fuel saving and less nuisances from aviation. Solutions are known collectively under AMAN acronym for Arrival Management.
As said, this fix is established on a route prior to the terminal airspace, 10,000 feet or more above the airport elevation. It can be the feeder fix.
The concept is to manage cruise speed to arrive at the metering fix on time instead of holding/vectoring before (or in) the terminal area. ATC can determine in advance times aircraft will be allowed into the TMA, based on aircraft capabilities, traffic and weather). ATC is able to issue an "EXPECT descent in n minutes" to facilitate planning by crew using their FMS.
This allows for limited vectoring and holding prior to the TMA. When holding/vectoring is still needed, it happens at higher altitude, which is better for noise/emissions management.

Thales AMAN/DMAN concept in Europe,source
For example, there is a metering for arrival at Sydney. Sydney has a curfew at night, aircraft are descended in the order they arrive, which encourage crews of early flights to arrive before the end of the curfew and hold. But because many crews use this trick, waiting time increases. By using metered arrivals (ALOFT in-house system and MAESTRO AMAN from Thales), crews are provided with a narrow time window for crossing metering fixes, one at 160 NM from the airport, the other at 40 NM. Once you have the window, there is no need to hurry and then wait. The time crossing the first metering fix can be provided up to 1,000 NM before arriving at this fix.

What's exactly a profile descent and how is it related to the metering fix?

The descent profile is the curve of altitudes followed for descent. The most efficient way to descend is the continuous descent without level segments. The aircraft just glides with idling engines and clean configuration (less noise, less fuel, reduced wake).
This profile is only possible if nothing prevents descent at any point, e.g. congestion in the terminal area. By ensuring a free descent after the metering fix, this portion of the descent can be optimized for fuel saving and noise reduction.

So how does a metering fix work to absorbs arrival delays?
What merits does it have as compared to delay vectors and holds?

The idea behind AMAN is to regulate traffic as far upstream possible, start sequencing early, and prevent wait time in flight. There is a complementary solution which is currently investigated too: DMAN for Departure Management, with transition fixes (also visible on the picture).
AMAN and DMAN systems can be coupled. As pointed out by @Lnafziger, metering fixes can be used farther upstream than the terminal area, up to departure. Such upstream fixes are visible on the picture. If we add Surface Management (SMAN), block-out times can be optimized effectively, minimizing taxi and airborne time, and at the same time minimizing delays and nuisances.
These systems tend to switch from distance sequencing to time sequencing. This field is currently being actively experimented, both in the US, Europe (LFPG experimenting coupled AMAN/DMAN) and Australia and a few other locations.

Source
For more, see also: Accelerating Air Traffic Management Efficiency: A Call to Industry
I hope @DeltaLima will show and provide additional elements for Eurocontrol.

Do you know what the metering fixes are at SYD?

@TomMcW: I don't know, maybe BOREE (339°/40NM from SY VOR) could be one of them (see TAC-4 chart).


text:
Does lift equal weight in level flight?
Does lift equal weight in level flight? Or does it equal weight + down pressure on the tail?
Added) Let's say the total weight of the aircraft on the ground was 100,000 lbs, and when it took off and leveled off, the down force from the tail was 10,000 lbs. Then what's the total lift required to support the level flight?

Lift should be considered as a whole, as the lift of the entire aircraft, wings, tail and body...

@xxavier Thank you for your reply but I'm not sure your comment is an answer to my question

That's why I wrote it as a comment, and not as an answer...

When you consider the aircraft as a whole, lift equals weight in level flight. This must be true following the laws of physics.
However, if you break down the forces, the lift produced by the wings does not necessary equal to the weight of the aircraft. Lift is also generated by the body (albeit very inefficient) and by the tail. Most horizontal stabilizers generate negative lift, but some (e.g. the A380) generate positive lift.
If we ignore the fact that fuselage produces lift, and assume a configuration where the tail produces downward force, then you are correct. But the fuselage does produces lift. Modern aircraft designs use software to take that into account.

Can you clarify "Modern aircraft designs use software to take that [fuselage lift] into account."  Do you mean the lift predictions in the flight control software use fuselage lift, or that modern aircraft shape and structural design includes calculations of fuselage lift?

"Most horizontal stabilizers generate negative lift" Why is that?

@asawyer so that if power is lost the airplane is in nose down attitude, which is safer, rather than nose up, which can lead to stall aka unsafe.

@vasin1987 Ah that makes sense. Thank you!

@CodyP the latter.

@vasin1987 I'm confused by your comment... if the horizontal stabilizer produces negative lift i.e. a downward force on the tail, wouldn't that result in an upward pitching moment?

@David yes it would cause upward pitching moment. However when airplane losing speed, say engine out, the upward pitching moment would be lesser make airplane to pitch down and gain airspeed.

Ah, when you put it that way it makes perfect sense.

@asawyer: Because of a nasty effect on stabilization. If the nose is up quite a bit, and the plane is flying slowly, the horizontal stabilizer may stall in the turbulent air behind the wing. With negative lift, the loss of negative lift helps to stabilize and level the plane. But if the horizontal stabilizer would have been providing positive lift, its loss would lead to the tail sinking even further, which might be unrecoverable.

From this discussion, I got several very important things:
1). Most horizontal stabilizers generate negative lift, but some (e.g. the A380) generate positive lift.
2). If power is lost the airplane is in nose down attitude, which is safer, rather than nose up, which can lead to stall aka unsafe
3). The fuselage does produces lift.
Thank you to all my friend here, especially @kevin and vasin1987. I do understand now why is recommended to put the fatter passenger in front seats and the slimmer in rear seats. We have to anticipate the worst, which to maintain the airplane still fly.

What exactly you mean by "lift" is important to answer this question. In unaccelerated flight (steady speed, constant rate of climb or descent, including level flight), all forces on the airplane are balanced - forward equals backward, up equals down, and left equals right. Weight is generally a downward force, lift upward, drag backward, thrust forward, and sideways force is generally minimal, but comes from things like rudder trim or the various forces involved in P-Factor (this also changes based on how you define "up", as in a turn lift will become a partially sideways force if "up" is defined by something outside the airplane, e.g. earth).
If by lift you mean the sum of all up/down aerodynamic forces (which is how I tend to think of it, in general, though this is not technically correct - see footnote or linked article) and by "up" you mean "directly away from earth's center of gravity", then yes, lift equals the aircraft's weight in unaccelerated flight. There may be some negative lift (as you say in your example, the 10,000 pounds of tail-down force), but this will be balanced by additional upward lift over the airplane's weight - that is, if the sum of all downward aerodynamic forces is 10,000 pounds and the aircraft weighs 100,000 pounds, then the sum of all upward aerodynamic forces must be 110,000 pounds. However, when speaking of the sum of forces, we must remember to treat them like the vectors they are - so 110,000 pounds of upward lift plus 10,000 pounds of downward lift equals 100,000 pounds of net upward lift - the airplane's weight.
If your definition of lift does not include that created by the engines (i.e., "vertical component of thrust"), or really, if you use any other definition without restricting it to special cases, then this may or may not be true - if the vertical component is zero (it's usually small, but probably not quite zero), then total lift will equal aircraft weight for unaccelerated flight. If it is nonzero, then lift plus the vertical component of thrust will equal weight, but lift alone will not equal weight in unaccelerated flight.
Footnote on the definition of "lift": the technical definition of the lift force is the component of force exerted on a body by fluid flowing past it which is perpendicular to the flow of the fluid - so my definition would include the drag of a falling body as a "lift" force, which it is not.

Lift is clearly defined. Why do you need to make up your own definition (which is neither logical nor intuitive) for it? Would you say the statement "lift needed to perform a 60° bank turn is equal to the lift needed for level flight" is true?

I'm not so sure lift is clearly defined. Sure, in the easy case of an infinite fluid flow without turbulence, the direction of fluid flow is well-defined. Reality is somewhat more complex, with wind and convection. And I'm not sure lift is even defined at all for that "falling body" example. There are two directions perpendicular to the fluid flow, after all.

Lift as a scientific term is clearly defined, but in practical terms for myself as a pilot, answering questions like this, that definition often leads to more confusion than clarity. Yes, the true lift forces on an airplane rarely equal its weight; a propeller's thrust, and much of a jet engine's as well, comes from the lift force, but it opposes drag instead of weight, and no one ever refers to thrust as "forward lift." It is much easier to speak of lift as "the force that opposes gravity" in many cases.

@Stankinator I have no problem using simplifications such as "thrust is parallel to aircraft axis", but referencing a flow magnitude (lift) to a random unrelated variable (gravity direction) doesn't make sense at all. How would you answer the lift statement question in my first comment? I really couldn't explain basic concepts such as "lift coefficient" or "increased stall speed in a turn" with your lift definition. Changing the definition depending on the question is for sure more confusing than just using the correct one.

@MSalters There are infinite directions (aka plane) perpendicular to the fluid flow and lift points in one of them. The good thing about having a consistent lift definition is that you don't have to change it depending on the flight attitude- it remains the same even in a nosedive in Mars. Admittedly, there are special situations where you need to define reference directions or quantities yourself, but it's not the case now.

@Stankinator, there are clear force direction. Lift force consists of two component: Lift force, which is L * cos (AoA), which AoA=Angle of Attack. This force is oppose the w=mg. While induce and parasitic drags are opposing the thrust. to make airplane fly (i:e: cruise), the L * cos(AoA)=mg. But, L * cos(AoA) itself is produced by the thrust which must be much greater than the drag components.

Short Answer: No
Long Answer:
In real flight the equilibrium of forces in the Z axis is a little bit more complex. You should consider also the effect of the different angles and component of Thrust as indicates the next Figure:

The equation should be:

If we consider static flight without normal acceleration and considering small values for the angles; the Lift Force would be $L = W-T*alpha$

Yes. Lift equals weight in level flight.

Answers should have a little background provided

Perfect answer.

You also forgot thrust. Particularly with vectored thrust at slow speed, you can get level flight with a high angle of attack/thrust vector and most of the weight is counteracted by thrust. e.g. this harrier transitioning to level flight

  or this park flyer struggling into a head wind at an almost vertical angle of attack (hence little aerodynamic lift)

 . Both of these are flying level with near to zero aerodynamic lift.
Net lift can be considered to be acting at 90 degrees to the velocity. Net lift includes contributions from wings, tail, canards, brakes, flaps and body, some of which may be acting upwards or downwards. Weight acts downwards. Thrust can acts at an angle to the velocity, either due to pitch or vectoring. So depending on pitch or vectoring, even if velocity is horizontal and net lift vertical, thrust can have a vertical component which acts against some of the weight.

I think the second video needs some kind of clarifying statement.  As it is, "struggling into a headwind" statement followed by "near to zero aerodynamic lift" could lead the unwary to believe that wind doesn't contribute to aerodynamic lift.

A better way to say would be that in case of a steady level flight, the aerodynamic forces (including the propulsive force) and the body force are in equilibrium.
The aerodynamic force includes the lift created by the wing, fuselage, the down force created by stabilizer etc. So, in your case, the lift equals the weight plus the tail down-force.

So if the total weight of the aircraft is 100,000 lbs and the downward force from the tail is 10,000 lbs, can I legitimately say that the total lift the aircraft is producing in level flight is 110,000lbs?

@lemonincider No, the total lift would be 100,000 lbs since it is the sum of all positive and negative lift vectors.

@SMS von der Tann Okay then let me put it this way. Let's say the total weight of the aircraft on the ground was 100,000 lbs, and when it took off and leveled off, the down force from the tail was 10,000 lbs. Then what's the total lift required to support the level flight?

The "down-force" the tail produces is lift - it just happens to be pointing downward.  Calling it anything other than lift is confusing.
In level flight, weight == net lift, where net lift == wing lift + body lift + tail lift.  If the tail lift is negative (which isn't true for all aircraft), the wing lift or body lift must be more positive to make up for it.
@lemonincider, in your example (wing lift + body lift) = weight - tail lift = (100k) - ( - 10k) = 110k lbs.

@lemonincider: Can we agree that the lift of the wing-fuselage-combination is 110,000 lbs but the total lift is 100,000 lbs?

I got it now. Thank you


text:
Is there a landing gear system that extends automatically to prevent belly landing?
To my knowledge, most aircraft with retractable gear are equipped with a landing gear warning system that makes repetitive beeping sounds in case the pilots try to land without the gear down.
I imagine big jetliners like Airbus and Boeing would have a far more advanced landing gear warning system, but does any of them have a landing gear system that extends automatically if the pilots fail to extend the gear and continue their descent for landing despite the warning?

As I remember, some models of the Piper Arrow and the Beech Sundowner had automatic landing gear extension, but I've never heard of such for large aircraft, and I don't recall any jetliner ever having made a belly landing because the the pilots forgot to extend the landing gear and ignored the warnings.

The danger introduced -- the system malfunctioning and extending unexpectedly inflight or extending during a ditching -- probably outweighs the danger mitigated -- pilots forgetting to extend the gear despite warnings.

What if the pilot actually wants a belly landing, like in the Miracle On The Hudson?

@ Koyovis: And particularly for aircraft other than commercial airliners, how do you reliably detect that a landing is intended, rather than just a desire to fly low & slow?

The proper question that should be asked before is: how many instances were there that a big airliner landed with gears retracted because the pilots forgot to extend them?

@Terry Not a full stop landing, but a belly touch and go by a jetliner due to pilots forgetting to extend: https://en.wikipedia.org/wiki/Mal%C3%A9v_Flight_262 So, it can count as both landing and take-off : D

@Agent_L Ah, yes, I had forgotten about that one. I don't know about Russian airplanes, but in the aircraft I flew, the only way you could turn off the GPWS was to pop a circuit breaker, and when pilots start popping circuit breakers (unless directed to do so when following an emergency procedure checklist) they better damn well know what they're doing, and be perfect in respects. Those boys obviously failed that test. LOL

@Terry: Found another one.

And then there's the problem of pilots flying amphibians putting the landing gear down for a water landing. Years ago on Fernridge Reservoir west of Eugene, Oregon an FAA inspector did that with a Lake Amphibian. They gave him an award (fake of course) for being quickest to the scene of an accident.

The simple fact is that an unintended gear deployment could be catastrophic, as it has a profound impact on the flight dynamics of the aircraft (one of the reasons there are warnings for both retraction and extension).
The benefits would be marginal as checking gear down is part of the bread-and-butter procedures for all retractable gear aircraft.
Keep in mind adding some automation to an aircraft has far ranging impact, just off the top of my head adding automatic gear mechanism would require:

Standardization of the automation
Modification of procedures
Training of pilots, and then re-certification of pilots
Training of maintenance crews, and then re-certification of the crews
Possible change to the maintenance schedule of aircraft
Certification of the new system by the regulator

and you would still need a backup / redundant system; and now you are back to the manual controls.
I can imagine this being implemented in remote piloted or autonomous aircraft though; as an additional workload relief.

An autonomous aircraft with retractable gear that lands automatically would need to be able to handle this anyway and will already have it. Most likely some custom design.

Yes, but I was assuming remote piloted aircraft - not autonomous.

Very good answer. I would add that in the event of a voluntary belly landing, it would be inappropriate to add workload into the cockpit just to prevent this "automatic landing extension" from occurring.

It has been tried before.  Piper's PA-28R and PA-32R aircraft were equipped with an automatic landing gear extension system which would automatically extend the landing gear below 85kts regardless of the position of the selector switch in the cockpit.  At higher airspeeds this system was overridden by a separate pneumatic system which deferred control to the cockpit selection handle.
This system did have its drawbacks, most notably being there are times where a pilot wishes to keep the gear retracted regardless of airspeed for performance reasons.  A fatal crash involving a PA-28 caused by an increase in parasite drag from the gear automatically extending caused Piper to subsequently issue an AD requiring all PA-28s and PA-32s with the automatic gear extension systems to be fitted with an emergency override, allowing the pilot to continue to command the position of the gear with the cockpit selector handle regardless of airspeed.
I suspect most GA aircraft and transport category airplanes never implemented such a device because the risks of an inadvertent gear up landing were outweighed by the advantages of having the ability to select the position of the gear for operations in all areas of the flight envelope.

While practicing stalls or slow flight an Arrow will drop the gear, unless you remember to hold the override in position.

Wow, so when landing (in a nice controlled fashion or otherwise) instead of the pilot choosing to lower the gear or not, the pilot can choose whether to: lower the gear, not lower the gear, allow the gear to lower on its own, not lower on its own but then lower it themselves, or not lower on its own but then continue to keep it up... Did I miss any?

The EMERG OVERRIDE lever has a manual locking pin on it which can be engage prior to flight.  Usually it is left engaged as such per the OEM checklist unless needed for an emergency.

If the pilot either forgets to command the gear down using the panel selector switch or willfully omits this task, once the airplane decelerates below 85kts, the gear will extend and the landing gear warning horn will activate, except if the throttle is wide open.  Also during takeoff if the EMERG OVERRIDE is engaged and the pilot commands the landing gear up using the panel selector switch, the landing gear will not retract until the aircraft accelerates past 85kts or the EMERG OVERRIDE lever is pulled up and locked in place.

My first night flight as a student was in a PA28, and the instructor neglected to tell me about that feature. We're climbing out, I'm utterly entranced with the moonlit night and... beep-beep-beep and the dash lights up - oh lord, something serious has broken. Nah, the instructor said, and pulled the override. From that, I learned two things: Know your aircraft before flying it, and carry extra underwear.


text:
How is the camber calculated?

In this picture, how do you determine the amount of the camber? Is it the average difference in the distances between the mean camber line and the chord line from the leading edge all the way down to the trailing edge?

The mean camber line is the locus of points halfway between the top surface and the bottom surface (which are sometimes referred as upper and lower cambers). For a symmetrical airfoil, it is merged with the chord line.
This curve is described by a polynomial function at each point along the chord axis. An airfoil may have a camber line which changes direction at one or more point, and which possibly crosses the chord.
All measures are taken perpendicular to the chord which is considered having a length of 1 or 100%.

Adapted from Wikipedia.
What you ask for is the maximum camber, which is determined by the value of the maximum difference between chord and mean camber line, and the distance from the leading edge.

Adapted from Wikipedia.

So it looks like there is no such thing as the "average camber." Then, when deployed flaps are said to increase the camber, what exactly does it mean? Does it mean the maximum camber increases?

@lemonincider: The "average" camber is likely the "mean" camber. when flaps are extended, the chord line changes because the trailing edge is lowered, this increases the maximum camber.

Is the mean camber different from the mean camber line? Then, is it the "mean camber" that is referred to when the flaps are commonly said to increase the camber?

@lemonincider: No, this is a shortcut, like chord line and chord.

I probably would have located the leading edge point of the chord line at the mid-point of the leading edge radius (slightly higher, in the "middle" of your drawing above). But the implication is that the leading edge chord line is anchored where the leading edge of the mean camber line starts. Is that correct?

@PeterT: Correct. Endpoints of both (mean) camber line and chord line are the identical. Mean camber line, near the leading edge, is the median of outer surfaces, it also where the stagnation point (the point where upper and lower streamlines separate) will be. This is more clear on turbine-blade-like airfoil.

@mins I thought I understood you but seems like I did not. Let me ask you once again. So when we say "this airfoil has a greater camber than that airfoil does," how exactly is it determined? Based on what? What's the criterion? The maximum camber? Or the average of the difference between the mean camber line and the chord line? Or something else?

@lemonincider: "this airfoil has a greater camber than that airfoil does" is not particularly clear, in particular if the airfoil has a camber inversion (reflex camber. What would that mean in this case?)...

... The more the airfoil is cambered, the more it is asymmetric , at some point the bottom face becomes concave (by definition of the mean line) and the chord is outside of the airfoil. So yes, when the camber is not reflexive (no inflection point), this is a measure of the difference with the chord.

@mins I came back to this question, because I came across the phrase "the flaps increase the camber" but couldn't understand exactly what was meant by that. Thanks.

@lemonincider: That's true, they move the trailing extremity of the chord line down, so the camber has increased (and the airfoil bottom is more concave).


text:
What's exactly meant by the "slippery when wet" report on ATIS?
I read this discussion and discovered the definition of "slippery when wet" from this advisory circular but I'm still having a hard time understanding why the runway condition is expressed as "slippery when wet" rather than just as "slippery." My guess is that the wet runway normally doesn't bring a slippery runway condition but it sometimes does and in that case it's expressed as "slippery when wet" rather than just "slippery." Is my guess correct?

@Federico Corrected

why the runway condition is expressed as "slippery when wet" rather than just as "slippery."

Because the runway is normally not slippery, and thus it cannot be described simply as "slippery", but it must be specified when this occurs, particularly when it is wet it is slippery.
Saying only "slippery" would mean that the runway is always slippery, and is not the case.
Saying only "wet" does not convey the information that it is also slippery in that condition. (and you want to specify this because not all runways are slippery when wet)
It is not said "slippery when icy" because "when icy" everything is slippery, so that does not need specification, "icy" is sufficient.
"slippery when snowy" I never heard of it, generally because either the runway gets snowploughed fast enough, or because when there is too much snow the runway gets closed.

I feel this answer is along the lines of this video: https://www.youtube.com/watch?v=3m5qxZm_JqM

Thank you for your reply. So if the runway is slippery due to ice, is it reported as "slippery when icy" and if the runway is slippery due to snow, is it reported as "slippery with snow"?

@lemonincider no, edited


text:
Is procedure turn considered published approach course?
Procedure turn is not completely lined on the chart and only marked with the barbed arrow. Is it still considered a segment of a published approach course? Can you report to ATC that you're established on the course when you're flying PT?

Jepp or Gov't chart? Could you provide example of the chart, or the name of the approach, and I'll attach it for you.

I think both charts use the barbed wire to mark standard PT.

I've only used Gov't charts. I'm not familiar with that term "barbed wire" as it relates to plates. I'd like to see what it looks like, maybe I just think of it as something else. Do you have an approach name, like an ILS or VOR etc and name of airport that I can go look up?

Sorry. I think the proper name is barbed arrow not barbed wire. My apologies

I’ve been doing a lot of practice approaches lately and if a procedure turn (or hold) is required, they tell me to 'report procedure turn (hold) inbound'. Then they clear me for the approach. I think this is why:
AIM 5−4−7. Instrument Approach Procedures

b. When operating on an unpublished route or while being radar vectored, the pilot, when an approach clearance is received, must, in addition to complying with the minimum altitudes for IFR operations (14 CFR Section 91.177), maintain the last assigned altitude unless a different altitude is assigned by ATC, or until the aircraft is established on a segment of a published route or IAP. After the aircraft is so established, published altitudes apply to descent within each succeeding route or approach segment unless a different altitude is assigned by ATC. Notwithstanding this pilot responsibility, for aircraft operating on unpublished routes or while being radar vectored, ATC will, except when conducting a radar approach, issue an IFR approach clearance only after the aircraft is established on a segment of a published route or IAP, or assign an altitude to maintain until the aircraft is established on a segment of a published route or instrument approach procedure. For this purpose, the procedure turn of a published IAP must not be considered a segment of that IAP until the aircraft reaches the initial fix or navigation facility upon which the procedure turn is predicated.


text:
What causes a slight increases in indicated stall speed with altitude?

Theorectically, the stall speed in IAS doesn't change with altitude, but this graph above shows the stall speed actually starts to increase a little from above a certain altitude. How can this be explained? Is it something to do with a compressibility effect?

The U-2C (which the graph is for) had a stall speed of about 79 KEAS, that is 79 knots equivalent airspeed (EAS) at all altitudes. At 50000ft, 79 KEAS is about 80 KIAS (indicated airspeed), but at 75000ft, 79 KEAS is about 83 KIAS. This is simply a consequence of the way that indicated airspeed is measured by pitot-static systems - the measured speed is not quite a true representation of the effect of the airflow upon the airframe. So, the apparent increase in stall speed is just an artefact of the way airspeed is measured, rather than the performance of the airframe itself.
You can confirm the EAS/IAS conversions for yourself using the Hochwarth Aviation Calculator.

Many thanks for the link to the Hochwarth Aviation Calculator!


text:
Do gliders make a better pilot?
I've often heard that learning to fly gliders makes you a better airplane pilot, in person and in answers here and here. Is this true, and if so, what specifically is it about gliders—or at least glider training—that makes this true?
Note: I understand that age, health and/or cost can make gliders more practical or accessible to some folks, but for the purposes of this question, assume those aren't a factor.

OK, my comments are gone, looks like you are getting a solid answer.

I may be biased because I learned in gliders, but, concerning how soaring biases students to be better "stick and rudder" pilots:

The obvious one that has been covered in the linked answer: gliders don't have an engine.  The result of this is that flying a glider is an exercise in energy management and a more direct interaction with your drag polar for a given configuration--because you don't have any way to add more energy to the equation.

In other words, your duration aloft, your ability to land in a particular location (of particular importance in a glider, since you only have 1 shot at landing and it could be unexpected due to variations in ridge lift, etc.) -- everything relies on flying the aircraft so as to manage the kinetic and potential energy of the aircraft to achieve a certain end -- and you don't have any "liquid" energy (e.g., fuel) to add to the vehicle.  Generally, you're trying to fly the aircraft such that it creates as little drag as possible, so turn coordination becomes more important (also can lead to spins, depending on condition), speed to fly for prevailing conditions and flight mode becomes really important.
The quintessential example of this (at least, the one that I have a video of) is from a student at my undergrad.  He was learning to fly one of the Grob 109b motorgliders the school owned.  As part of the class, the students demonstrated their skill at managing the aircraft's energy by starting, with the prop off, 1000' above ground and maybe 600' from the end of the runway (horizontal distance).  Within 15 seconds, the aircraft would be on the ground just a little way beyond the runway numbers.  And you have to do it without exceeding the aircraft's maneuvering speed during the vertical dive, conserving enough energy to make the runway, but not too much energy (else you'd just float the length of the runway).

Another facet of the above: increased adverse yaw effect.

In a glider, keeping the yaw string straight (e.g., maintaining coordinated flight) is constantly drilled into you as one of the most important things you will do (besides flying the airplane).  I remember flying one of my last instructional rides before my checkride and my instructor walked up with a sheet, taped it over what instruments the glider did have, and said, "Let's go flying".  The only instrument I could see was the yaw string, and the point was that that was "all" I needed (I had a feel for airspeed based on pitch).
What the yaw string does is tell you whether or not you are flying straight (i.e., what direction the relative wind is going relative to the aircraft).  As noted before, this is quite important, because, if the relative wind isn't parallel to the longitudinal axis of the aircraft, your drag is going up.  Gliders are, typically, more prone to slipping or skidding (i.e., flying at angles to the relative wind, particularly in turns) due to their long wings, which, when they have an aileron deflected, can induce a greater yawing moment than your typical general-purpose aircraft with a much lower aspect ratio wing (i.e., a more shorter, stubbier span, whereas gliders are made with long, skinny wings to decrease induced drag).  This makes the aircraft's yawing moment more sensitive to aileron deflections and  more prone to fly in an uncoordinated fashion--hence instructors make it a point to train glider pilots to pay more attention to how coordinated their vehicle is at a given point in time.
This is of particular importance, not just for drag, but also for flight safety.  If you have a glider that's low, relatively slow, and making a steep turn while landing (read: flying at a high load factor, thus increasing stall speed), an uncoordinated turn can do this, or stall, spin, and crash due to insufficient altitude for a recovery.  It's a really big deal, and, again, you don't have the ability to just "add power" when you feel like you're getting slow.

Situational awareness

One trick with gliders is that you are very dependent on the atmosphere.  Say you are trying to get from point A to point B, say for a race or what have you, and you planned out your flight to run a ridge based on forecasted winds.  You're up, running the ridge at 200' (or less) above ground level, and the wind peters out and dies -- you're stuck.  When I learned to fly ridges, the mantra was to always have a place picked out such that, if the wind died right now, you'd have a place to land.  200' was relatively high among the folks I knew flying the ridge (most guys would be flying at least half that, and I heard stories about folks flying at 20 feet above the trees), and that doesn't give you a lot of time to figure out where you are going to land...and landing out (i.e., not at an airport) is not uncommon.  Glider pilots train for it, it's part of our ground school, and it's beat into us while flying.  If you're taking off via aerotow (i.e., being towed to altitude by another aircraft), the question during training is always, "If the rope breaks right NOW, where do you land?" Because you're getting energy from the towplane burning fuel on the other end of the rope, and, as soon as it's broken, you have not much time, some speed, and usually very little altitude to get safely to the ground in.
So, in the end, I'd think that glider pilots would have some edge in powered pilots in that regard.  Yes, powered pilots train for engine outs, but, in a glider, every flight is an engine out situation and you're flying at the mercy of the weather, how well you planned ahead for it, and how well you continue to plan while you're flying.  As quoted from an AOPA article:

U.S. Air Force Capt. Danny Sorenson, who instructs in F-16s, is a glider pilot. He stated, “As a result of my glider training, I’m always thinking, ‘Where can I land this thing?’” He also noted that during his F-16 training, simulated flame-outs were never a problem for him, “It’s instinctive,” he said. “I’d just fly my pattern and glide in.”

Summing it up:
Gliders are much "simpler" flying: there's not a VOR in a glider, no ILS, no autopilot...all you have to do is fly the airplane, and do it well.  And, if you do it well, the airplane flies well.  Do it poorly and you might die (if it's that bad, since you don't have 2nd chances in important things, like landing) or you just might not be in the air for very long.  It's a much more direct connection between the pilot's stick/rudder inputs, the vehicle's performance as an aerodynamic vehicle, and how the pilot wants to use the aircraft, though, and I think that's the key...and all of these skills translate to other aircraft (even rotary wing, as I found out when flying those once).
Sources:
--My experience (PPL in gliders)
--https://www.aopa.org/news-and-media/all-news/2002/may/flight-training-magazine/gliding-your-way-to-a-pilot-certificate
-- https://www.aopa.org/news-and-media/all-news/2009/january/22/training-for-that-moment-when-every-second-counts

In other words, in part, you really need to learn what to do with your feet.  Nice well thought-out answer.

I read this mostly that it's about learning to use your head


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What's required for the ADS-B to function? A GPS or Mode S Transponder?
Jeppesen defines the ADS-B as a surveillance system broadcasting GPS-derived positions and other information such as velocity over the data link. Yet, the Wiki article on ADS-B relates it to the Mode S transponder system. So the ADS-B operates either on a GPS or on Mode S. Is my understanding correct?

Your ADS-B transmitter gets its position data from a GPS source. Then it transmits the data using either 1090ES or 978 Universal Access. Mode S transponders already transmit detailed information about the aircraft, called a 'squit'. Some can be upgraded to send 'Extended Squitter' info. The other alternative, in the US only, is to transmit the info on the 978 band. More info at http://www.garmin.com/us/intheair/ads-b/squit/

@mins What I meant is closer to "What's required for the ADS-B to function? A GPS or Mode S Transponder?" I'll change the title

ADS-B, Automatic Dependent Surveillance - Broadcast, is a  broadcast of the aircraft's position and other information.
The position is determined by a GPS receiver. It is possible to use another qualified position source, but today it is de facto GPS. Other GNSS (Galileo, GLONASS, Bei-Dou) are other likely candidates in the future, but as far as I know, none have been certified for ADS-B yet.
The broadcast is usually by the Mode S transponder but can also be done by a dedicated transmitter / transceiver in the Mode S format and frequency (1090 MHz).
In the USA below 18 000 ft, in addition to the Mode S based ADS-B, a system based on Universal Access Transceiver  (UAT)  is used. This operates on 978 MHz
A third broadcast system was VDL mode 4, it was never rolled out on large scale.
So ADS-B uses GPS to determine the position and it operates either  on 1090 MHz (a frequency also used by Mode S)  or 978 MHz to broadcast information.

Then in order for the ADS-B to function, an aircraft should be equipped with both a GPS and a mode S transponder (or UAT). Without either of them, the ADS-B simply doesn't function. Correct?

You could have an ADS-B transmitter on 1090MHz that is not a Mode S transponder, but that is unusual.

Okay I'll take it as yes. Thank you for your detailed explanation.

Nothing requires the position to be determined by GPS specifically. Requirements are just for precision and any GNSS can do that, so GPS, GLONASS, Gallileo (and BeiDou-2 is under construction and some more are planned). Since the principle and protocols are similar, modern receivers may use more than one.

@JanHudec you are right of course, but effectively only GPS is used at the moment.

@DeltaLima, also, even if there are already dual-system chips in use (they are in use in handheld devices for quite some time, but certification for aviation takes time), I doubt any chip would only use some other system and pass on GPS.

The other thing is, however, that it's still quite common to see aircraft report just INS position, which shows on FR24 as aircraft landing say a mile off the runway.

I believe that the position can be from any approved position source, which just happens to be GPS. In theory one could derived the position from a non-GPS source such as Galileo, GLONASS or INS/IRS if it meet the position source requirements. See https://www.ecfr.gov/cgi-bin/text-idx?node=14:2.0.1.3.10#se14.2.91_1227

FYI, some older plane's have a waiver to transmit a position derived from non-GPS (usually DME/DME/IRU) sources, because there is no certified GPS available for those types.


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Is it common for jets not to have backup manual control system for emergencies?
In July 1989, the United Flight 232 crashed in Sioux City, Iowa, while the pilots were trying to land on the runway without any yoke control. Today, I was reading the PHAK and found this phrase: "In the event of complete (hydraulic or electrical) power unit failure, movement of the control surface can be effected by manually controlling the control tabs" (4-46).
Why didn't the pilots of the United Flight 232 have a backup manual control when their hydraulic system failed? Is it because the aircraft (DC-10) was equipped with three independent hydraulic systems, which were considered to provide adequate redundancy for emergencies? Then, what about other modern passenger jets like Boeing and Airbus models? Do they have a backup manual control system in case of such a hydraulic or electrical control system failure?

Boeing 737 has manual backup. After UA 232, hydraulic fuses were installed in most aircraft types, not in DC-10s though.

@Koyovis Yes I found that out later while studying the role of control tabs on the wings, which function as servo tabs in case the pilots have to fly manually. Thanks

On any large jet (the DC-10 was a wide-bodied plane, seating around 300 people), the control surfaces are simply too large to move without hydraulics. Manual control is impossible, and multiple redundant hydraulic systems are provided. This applies to any large plane.
It's considered unlikely that all the hydraulic systems will fail independently. The specific problem on United flight 232 was that the failures weren't independent: all three hydraulic systems were severed by the engine in the tailplane failing and spewing lumps of metal into the narrow conduit that carried the hydraulic lines.

If a system fails catastrophically enough, it doesn't really matter how many redundancies are installed.

@hBy2Py That statement carries almost no information: it's a definition of "catastrophically enough" and it doesn't say anything about whether "catastrophically enough" is even possible. For example, the third engine of UA232 failed catastrophically enough to take out all three hydraulic systems, but it didn't fail catastrophically enough to take out the other two engines.

In one comment @DavidRicherby sums up a large part of engineering.

It's probably worth noting that the exact same damage that severed the three hydraulic systems would also have severed any backup manual control system if one was installed there.

@Peteris Honestly, I don't think that is worth noting. First, it's not necessarily true: a punctured hydraulic system is busted, whereas a partially cut cable is still a cable. But, most importantly, the lack of a mechanical backup had absolutely nothing to do with the possible failure modes of such a system. There was no mechanical backup because, even when in perfect working order, it wouldn't enable the pilots to move the control surfaces.

@DavidRicherby Adding to your comment... Encased cables can easily fail stuck, especially when partially cut, meaning it won't matter how many redundant controls you have if that cable is jamming the system.  And cables sticking is actually the most common failure mode for them, and is vastly more common than hydraulic faults.  If you're adding backup controls, the very last thing you want is for your backup to be less reliable!

@DavidRicherby Some jokes are pretty information-free, yes. If you want to flag that comment as too chatty, please do.

In response to the accident, they added "fuses" to the hydraulic lines to stop them from bleeding dry and manufacturers did a better job to route hydraulic lines along different runs so that one failure doesn't take them all out.

@Gray That's reassuring. This accidnet was not wasted. Thank you.

@lemonincider - very, very few are. The entire airline industry does its best to learn from every accident, especially the fatal ones.

@FreeMan: And almost all of the ones they don't learn from are ones where they never found out what caused the accident in the first place, so they don't have any way of knowing how to keep it from happening again.

@Peteris: Even if all the cables to the tail were severed, the aileron/spoileron part of our hypothetical DC-10 manual-reversion system would still have been left fully intact (which would have greatly eased control of the aircraft, by allowing mechanical aileron/spoileron control and allowing the pilots to dedicate the throttles to controlling altitude instead of also having to simultaneously throttle-steer the aircraft), unlike the hydraulic situation.


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Do airlines in Middle East use higher cost indexes?
Cost index equals time cost divivded by fuel cost. Given that, do the airlines in Middle East generally use higher cost indexes because fuel costs are much cheaper?

For those wondering what a cost index is, we have a question for that: https://aviation.stackexchange.com/questions/1984/what-is-cost-index

"Cost index equals time cost divivded by fuel cost" - Is this an assumption or fact? Do you have a source?

@BDLPPL: There are detailed explanations of cost index, e.g. by Airbus (§2.3) and Boeing (first page). They are fully in agreement with the OP.

@mins Perfect, just what I was looking for!

@mins Thank you so much for the materials :)


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What do these numbers and symbols on the ICAO SIGMET chart represent?

Anybody who can identify the numbers and alphabets in the red, green, and purple circles on the chart above?


text:
What's the relationship between drag divergence Mach number and maximum operating limit Mach number?
The drag divergence Mach number, also known as the force divergence Mach number, is the Mach number where drag waves start to form. The drag divergence Mach number is typically between 5 to 10 percents greater than the critical Mach number.
The maximum operating limit Mach number(Mmo) is the mach number beyond which aircraft start to suffer compressibility effects and flutters at high altitude. It's basically the high altitude version of Vne(Vmo).
My question is: what are the two Mach numbers' relationships? Is Mmo always higher than the drag divergence number? If so, how much? Or is it safe to assume that they are more or less the same?

They are unrelated definitions. Mmo is set by the designer with input from actual flight testing, Mmo is a maximum safe speed and is only concerned with practical operation and structural integrity. Mach divergence is a physics phenomena. Mmo can be much lower or higher than the Mach divergence from one aircraft to the next. Mmo has a similar purpose to Vne but Mmo is a more useful measure for high performance aircraft.
It just happens that most commercial and large business jets are designed for high subsonic cruise so the design goal is to push divergence to higher speeds for more efficiency and Mmo ends up very close to Mach divergence.

This is very true, having said that though, I would have guessed that almost always Mmo will be below drag divergence mach number, as the shockwave formation and flow separation that leads to the precipitous rise in drag also leads to problems with wing flutter and the degradation of structural margins.

@AlastairWyllie All aircraft that fly transonic and supersonic must have mmo above mach divergence by simple definition.


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What drives N1 during a jet engine start?
A typical jet engine start sequence that I understand is as follows:
ENG Start Switch to Ground → Start Valve open → N2 increases → N1 increases → ENG Start lever to Idle (when N2 has reached a certain number) → Fuel flow starts → EGT rises → ENG Start Switch cutout
As far as I understand, N1 and N2 are mechanically seprated. If that's the case, what drives N1 before it starts to rotate from the following combustion?

N1 is driven by the starter flow through windmilling. N2 is started through a gearbox as described in this answer, driven by an air starter or by an electric motor. This causes air to start flowing through the engine, and the LP turbine blades pick up energy from this airflow.

Above graph is from The Jet Engine by Rolls Royce, and shows that the LP spool (N1) starts to turn in the dry cranking (starting) phase, after N2 has been accelerating for a while. The speed with which N1 spools up depends on the engine type, but even the large inertia of a helicopter rotor is slowly starting to be driven around by the starter flow.

I would have thought that N2 is driven through a gearbox receiving power from an air-starter, which receives the compressed air instead. Seems inefficient to try to drive the turbine directly with airflow. Are there different systems for engines of different sizes?

@aerobot Yes indeed! Have amended the answer.


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What error is left uncorrected when "fast IRS align" is done?

As far as I understand, there are two ways to align the IRSs in B737s: "Full align" and "fast align."
For a full alignment, you rotate the IRS selector from OFF to NAV, and for a fast alignment, you roate the selector placed at NAV from ALIGN back to NAV.
The 737 FCOM explains that a fast alignment "automatically updates alignment and zeros ground speed error." What error is left uncorrected when "fast IRS align" is done as opposed to when a full alignment is made?

IRS = Inertial Reference System for those like me who had no idea what IRS meant ;)

Your question presumes that a "fast align" can be performed instead of a "full align", but is in some way deficient. But this is not the actual usage.
Starting from cold (unpowered), the IRS must always go through a full alignment to find true north, a process which takes about 7 minutes. The IRS is also manually given an initial position, usually via the FMC 'POS INIT' page. From then on, the IRS will maintain its sense of true north, but will gradually drift in position and ground speed due to the slight errors in the accelerometers. The amount of drift is small enough not to be a significant problem even for the longest flights of which the aircraft is capable, but the IRS should be re-aligned to a known state and position before each new flight. During a turn-around, the IRS could be given a full alignment, but since it is already aligned to true north, that is actually unnecessary - all that does need to be done is to reset the accelerometers to zero when the aircraft is actually stationary and give the IRS a new initial position. That is the purpose of the "fast align". The result should not be in any way 'deficient' from a full aligment, it is just saving the unnecessary and time-consuming step of determining true north when the IRS already has that alignment.


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Do commercial jets ever make 360 in the pattern for separation?
It's not uncommon for GA aircraft to circle in the pattern for separation purposes. Does that ever happen to big commercial jets?
Imagine an airliner were cleared for a visual yet forced to execute a go-around and instructed by ATC to take a pattern. When there's another traffic on final, is it a viable option for ATC to instruct the jet to make a left or right 360 for the traffic on final? Does it actually happen in the real world?

Yes, it can happen, but it's pretty uncommon.
Airliners arrive at the destination airport having been sequenced by ARTCC ("Center") and Approach so that they are showing up in an orderly sequence, spaced out so that there is adequate room between them.  They may be set up for an ILS, or they may see the field and accept a visual approach, but either way the flow is generally quite predictable and orderly.  VFR traffic, by its nature, is less controlled and ordered, so it's more likely in that sort of an operation that you'd have two aircraft in positions to arrive at the runway at about the same time, so a solution like a 360 on downwind or on final becomes necessary.  With a flow of airliners (and other IFR traffic) spaced out at 3-5 miles apart, that solution would create as much trouble as it would solve, because now the conflict is with the aircraft behind the one that circled, rather than with the one in front of it.
What does happen more frequently, is the 360 happens much farther out from the airport, and that's called "going into holding" due to traffic saturation.  Maybe the airport is over-loaded with arrivals, or it had to suspend operations while the active runway was changed from 18 to 36 (or whatever they are at a particular field), or there is weather affecting the approaches which slows down the rate at which it can accept arrivals.  The Approach and Center controllers will work to keep the flow of traffic coming into the airport neat & orderly, which means delaying aircraft before they get to Tower's airspace (i.e. the "traffic pattern").  Ideally, they will do this with speed adjustments to get the desired interval, but sometimes the delay needed is more than speed adjustments alone can accomplish.  Then they'll put the arrivals into holding for however long it takes until they can start sending them in again.
That's the more common airline version of a "360 on final"... doing 360's with 10 mile legs on a really, really extended final -- 100+ miles out!
In the case of a go-around that turns downwind, rather than a 360 on final, the typical solution would be to extend turning final out until there is adequate space.  Staying on downwind past a 5 or 10 mile base & final is highly undesirable for a VFR pattern, but for airliners where everybody has an ILS and doesn't really have to maintain sight of the runway, it's no big deal.  For big airports, extending the traffic pattern out to longer finals is highly preferred over having jets circling on final.

I was speaking to a controller about this just recently, who said they much prefer to give a plane long slow S-turns to increase their spacing rather than a 360.   You can fine-tune spacing with an S-turn, but once you start a full-circle turn, you have to finish it, and if it takes too long, then you have more spacing than necessary.

As a passenger, I've seen it twice. Once, landing at JFK on an MD-11, it was already in final, full flaps, landing gear down, when the pilot applied full power, landing gear up, flaps up, completed the 360, flaps down, gear down, and then re-entered the final and landed. An, another time, on a CRJ700 at a small airport.

It can happen in China. I've heard of cases where ATC ordered a widebody to do a 360 to stay clear of other traffic at cruise level. Didn't get another flight level but had to fly a 5 minute 360 instead.

Actually a standard 360 degree turn will take exactly 2 minutes, not 5. A standard rate one turn is 3 degrees per second.

@J.Hougaard Talking about "at cruise level" I suspect he means a 5 minute holding pattern. Not what you & I think of as "a 360" but the distinction between a racetrack vs a continuous turn may often be lost on pseesengers.

@RalphJ I agree, but I thought it was worth clarifying regardless

@j.hougaard Jet aircraft at cruise levels will typically be bank limited and incapable of performing standard rate turns. Even if not incapable, half standard rate is more typical at cruise, at least for my operation.

@JWalters Exactly, for those looking for more info, see: What is the standard rate of turn for heavy aircraft?


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Does uncoordinated control affect stall speed?
Does an uncoordinated turn increase stall speeds? When you do cross control for crosswind landings in GA aircraft like C172, should you be worried about higher stall speeds during flares?

If you are taking flight lessons have your instructor demonstrate a cross-control stall at altitude. You'll be amazed at how much the higher the stall speed is than normal.

First question, yes.  For two principle reasons: instrument error and a change in relative wind across the wing. The increase in indicated AS would be small.
Second question, not a factor.  During the flare, you are close to the ground, and the AOA is mostly controlled by feel, sink rate and all the other things that factor into that complex maneuver.  So if the speed were slightly higher, it probably wouldn't make a difference.
But let's talk practicality.  When landing a C172 in a direct crosswind of 20k or so, I would add 5k to my speeds, and if the winds were gusty I would add accordingly (like 1/2 the gust factor approximately).
It is better to focus on having a stable approach with good AS and pitch control, than it is to fret over a potential 2k indicated stall speed differential.  Besides, in ground effect the numbers change as well.

Thank you for your reply. One follow-up question. What about during turns in the traffic pattern? Let's say you overshoot the final and use greater than 30 degrees of bank without stepping on the rudder to get back to the center line. Does this uncoordinated turn increase the stall speed?

@lemonincider Yes, that is the classic formula for a stall-spin on turn to final. If you find yourself in that situation, GO AROUND!

First, all turns should be coordinated, such as base to final.  Second, if you need more than 30 deg bank on final, you are likely exceeding the crosswind capability of the aircraft.  I teach a rather tight final, but if you are turning a longer final, I would do coordinated maneuvering to get you on the centerline, and then begin your uncoordinated final approach slip.  If you get messed up on shorter final, it's go-around time.

Yes that's for sure. But what I'm asking is not if you get into a stall or spin when your turns are uncoordinated; when you bank too steeply, you can fall into a stall or spin because of the resulting increased load factor. I'm well aware of that. What I'm asking is rather whether uncoordinated turns themselves affect the stall speed of an aircraft.

An uncoordinated turn changes indicated stall because of instrument error, and the relative wind change across the wing.  A load factor, such as from a turn can also change the stall speed, but I initially didn't address that due to the assumption that you were concerned about final and flares.  A crosswise flow will reduce lift, and in some aircraft it can be 40%.  With a reduction in lift, the stall speed increases. The crosswise flow will exist in uncoordinated flight, whether you are turning or not. If you are turning there will also be a load factor which will impact the stall speed.


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What are ATC's considerations when they instruct to intercept final approach course instead of simply clearing for approach?
When ATC provide radar vectors for an instrument approach, they sometimes instruct pilots to intercept the localizer or final approach course before giving an approach clearance, and other times give an "early" approach clearance with the final vector for the interception while the aircraft is still heading to intercept the localizer or final approach course. What are ATC's considerstions when they "delay" the approach clearance until the interception of the localizer or final approach course?

The main reasons I will instruct a pilot to intercept and follow the localiser instead of clearing him for the ILS are separation and minimum safe altitudes.
In a busy approach environment, it is often beneficial to let two aircraft intercept the approach with 1000 ft vertical separation. Maybe the aircraft are slightly too close horizontally but the first one is faster than the second one. I can then have then both follow the approach course, and descent number two on the approach once horizontal separation exists.
As for minimum safe altitudes, if you are approaching an airport on a heading straight in toward the runway, maybe there is room for you to intercept the approach course a large distance from touchdown. However, at this distance it might not be possible to descend to the approach altitudes, due to terrain or airspace restrictions. In this case I will also clear you to follow the localiser and then give the approach clearance later.

I'd also add the times where you need to intercept the approach as they're getting close, but you have to do something else, most likely base the next aircraft

It also seems that if they are doing visual clearances they often join the loc before the pilots can see the airport. I've always presumed they can't clear a plane for visual until they report they have the field in sight. Is this true in your jurisdiction? [You're in EASA land iirc]

@TomMcW I've never used a loc in combination with a visual. A visual approach is approved once the field is in sight

At large (class B) US airports in good weather, APP will often tell aircraft to intercept the LOC and report field in sight, and then they'll clear for visual approach, which allows lower separation and thus a higher arrival rate.


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What's exact offset degree dividing localizer and LDA?
When a localizer is offset at an angle with the runway track, it functions as a "Localizer Directional Aid or LDA." What's the exact offset degree dividing them? Any offset degree greater than 0 makes a localizer an LDA? I've checked various sources including AIM, but none of them gives me a definite answer.

That confuses me more. I guess FAA and ICAO simply have different standards for this

Here's an approach in KSFO. It's only 3 degrees offset but an LDA approach

@mins I think you have the right answer, at least for the FAA standards. Thanks!

@mins -- write it up and I'll give you an upvote :)

The maximum angular offset for a LOC is 3° for FAA and 5° for ICAO. Everything else is named either LDA (FAA) or IGS (ICAO).

In ICAO wording, any ILS which doesn't meet all ILS requirements is an instrument guidance system (IGS).
Localizer type directional aid (LDA) is a designation used by FAA.

Guidance /aid refers to the fact that this system is not a precision approach, only an assistance for for the approach.
The most infamous IGS (at least for sim users) is probably the one used at Kai Tak RWY 13 until the airport was decommissioned. Current ones include Sion LSGS RWY 25 which terminates before the runway (see also Washington KDCA RWY 19 as commented by @Gerry), Innsbruck LOWI with a 5° offset localizer or Lugano LSGS RWY 01 for its 6.6° glideslope.
Many of those non-standard approaches are required to access an aerodrome at the bottom of a valley. All require special training and certification before a pilot is authorized to use them.

ICAO defines approach characteristics in Doc 8168 Procedures for Air Navigation Services (PANS-OPS), volume II: Construction of Visual and Instrument Flight Procedures. According to SKYbrary (Eurocontrol) "PANS-OPS procedures are the international standard and are used throughout Europe and in many other countries world-wide".
FAA equivalent are the United States Standard for Terminal Instrument Procedures (TERPS, Order 8260.3). According to the same source: TERPS are used by "USA and certain other countries. These include Canada, Korea, Saudi Arabia and Taiwan".

ICAO PANS-OPS
Final approach segment:

(5.2.1) When runway aligned track guidance is not possible it may be
  offset up to 5 degrees without OCA/H penalty (see 5.4.3.1, “Aligned
  straight-in approach”). Beyond these limits (or where other
  requirements cannot be met) a circling approach shall be used.

ILS Cat I:

(2.1.1) The localizer course line shall intersect the runway extended
  centre line:
a) at an angle not exceeding 5°; and b) at a point where the nominal
  glide path reaches a height of at least 55 m (180 ft) above threshold.
  This is called intercept height.
(2.1.2) The procedure shall be annotated: “localizer offset ...
  degrees” (tenth of degrees). The general arrangement is shown in
  Figure II-1-2-1.

FAA TERPS
Localizer without glide path. Final Segment alignment (8-1-2):

Localizers aligned within three degrees of the RCL are identified as
  localizers. If the alignment exceeds 3 degrees, they will be
  identified as LDA facilities.

Localizer with glide path (ILS). Final segment alignment (10-2-2 a):

Where a unique operational requirement indicates a need to offset the
  course  from RCL, the offset must not exceed three degrees. The offset
  course must intersect the runway centerline at a point 1100 to 1200
  feet inside the DA point (see figure 10-2-2 ). For offset courses the
  minimum HAT is 250 feet and the minimum RVR is 2400.

For LDA with GS (10-2-2 b):

The final course maximum offset from RCL extended is 15 degrees. The
  final course must cross the RCL extended at least 3000 feet from LTP,
  but no more than 5200 feet from LTP.

KDCA Rwy 19 has two LDA approaches.  LDA Y RWY 19 has an LDA offset of 37.49 deg and a glide slope.  The GS is unusable below 1100' which leaves the DA at 1100' and 3.4 NM from the threshold.  LDA Z RWY 19 has an LDA offset of 39.99 deg with no GS. It has an MDA of 720' for the S-LDA approach with a MAP 0.8 NM (DME 1.6) from the threshold. This would appear to the extreme case for an LDA approach.

@Gerry -- never mind that one of the LDAs (the one that was called the ROSSLYN LDA RWY 19 in a previous life, I believe) is actually on top of a building in DC, not at the airport!

@UnrecognizedFallingObject  The localizer for the LDA Z RWY 19 is across the Potomac at Joint Base Anacostia-Bolling.

@Gerry -- a-ha, guess I'll have to give finding the antenna on Google Maps another shot then xD